Fetch: parse JSON off-main-thread (async) based on response Content-Type (#45188)

public/app/core/services/backend_srv.ts

@@ -20,7 +20,12 @@ import { getConfig } from 'app/core/config';
 import { DashboardSearchHit } from 'app/features/search/types';
 import { FolderDTO } from 'app/types';
 import { ContextSrv, contextSrv } from './context_srv';
-import { parseInitFromOptions, parseResponseBody, parseUrlFromOptions } from '../utils/fetch';
+import {
+  isContentTypeApplicationJson,
+  parseInitFromOptions,
+  parseResponseBody,
+  parseUrlFromOptions,
+} from '../utils/fetch';
 import { isDataQuery, isLocalUrl } from '../utils/query';
 import { FetchQueue } from './FetchQueue';
 import { ResponseQueue } from './ResponseQueue';
@@ -187,7 +192,9 @@ export class BackendSrv implements BackendService {
       mergeMap(async (response) => {
         const { status, statusText, ok, headers, url, type, redirected } = response;
 
-        const data = await parseResponseBody<T>(response, options.responseType);
+        const responseType = options.responseType ?? (isContentTypeApplicationJson(headers) ? 'json' : undefined);
+
+        const data = await parseResponseBody<T>(response, responseType);
         const fetchResponse: FetchResponse<T> = {
           status,
           statusText,